There is a continuing need for monitoring levels of liquids stored in closed containers in a noninvasive manner and without regard to the type of liquid being stored. Existing acoustical methods for level determination rely on a localized pressure disturbance generated using a transducer, and propagated through the storage material. The disturbance is reflected from the surface of the liquid and received near to the original transduction point. The period between transmission and reception is measured and the liquid level determined by the equation:
                              Level          =                                    c              ⁢                                                          ⁢              Δ              ⁢                                                          ⁢              t                        2                          ,                            (        1        )            where c is the propagation speed of a sound wave in the liquid and Δt is the amount of time elapsed between the generation and reception of the sound disturbance. The factor of two is a result of the sound disturbance traversing the liquid column twice: transmitter to reflector, and reflector to receiver. Pulse-echo, pitch-catch, and tone-burst methods of acoustic ranging are included in such measurements.
U.S. Patents for acoustic level detection in the time domain include: U.S. Pat. No. 6,427,532 for “Device For Measuring A Fill Level Of A Liquid In A Container.” which issued to Dieter Keller on Aug. 6, 2002; U.S. Pat. No. 5,99,407 for “Multi-Frequency Ultrasonic Liquid Level Gauging System,” which issued to Martin Hewitt on Dec. 7, 1999; U.S. Pat. No. 5,877,997 for “Pulse Echo Distance Measurement,” which issued to Roger Fell on Mar. 2, 1999; U.S. Pat. No. 5,827,943, Gazis for “Method For Calibration In Level Measurement,” which issued to Helmut Schmidt on Oct. 27, 1998; U.S. Pat. No. 5,793,705 for “Ultrasonic Liquid Level Gauge For Tanks Subject To Movement And Vibration,” which issued to Denos Constantinos Gazis et al. on Aug. 11, 1998; U.S. Pat. No. 5,697,248 for “Liquid Level Sensor,” which issued to Richard Hunter Brown on Dec. 16, 1997, and U.S. Pat. No. 5,400,376 for “Ultrasonic Fuel Gauging System Using State machine Control,” which issued to Christian L. Trudeau on Mar. 21, 1995.
A Frequency Modulated Continuous-Wave (FMCW) process using pseudo-frequency domain level detection is described in U.S. Pat. No. 5,799,534 for “Procedure For measuring The Level Of A Liquid In A Tank According To The Radar Principle,” which issued to Ronald van der Pol on Sep. 01, 1998. This technique is used in radar range-finding, and the liquid level is determined by observing reflections from the liquid surface of ultrasonic signals generated outside of the liquid.
In U.S. Pat. No. 6,053,041 for “Noninvasive Method For Determining Liquid Level And Density Inside Of A Container,” which issued to Dipen N. Sinha on Apr. 25, 2000, flexural acoustic waves are generated in the shell of a container holding the liquid and the phase difference between the detected flexural wave propagated through the shell from that of the originally generated wave is measured a chosen distance from the location of the generated wave. The generation and detection means are moved over the outer surface of the vessel, and a change in the phase difference indicates that a liquid/vapor interface has been crossed, thereby identifying the liquid level. In another embodiment, continuous liquid level measurements were made using an apparatus where one transducer was disposed near the top of the container, and a second transducer was disposed near to the bottom thereof. The time of flight of a tone burst transmitted from one transducer and received by the other was found to change linearly with liquid level since the sound speed of the wall flexural mode depends on whether the wall is in contact with a liquid. The liquid level was obtained by calibrating the time of propagation of the flexural wave as a function of the amount of wall exposed to the liquid.
In U.S. Pat. No. 5,767,407 for “Noninvasive Identification Of Fluids By Swept-Frequency Acoustic Interferometry,” which issued to Dipen N. Sinha on Jun. 16, 1998, a method for noninvasive identification and monitoring of chemicals in sealed containers is described, where direct access to the chemical is not possible. External transducers are used to introduce a standing-wave ultrasonic vibrational pattern into the fluid over a range of frequencies. Liquid characteristics are determined from the peak spacing in the interference pattern, the peak attenuation, the peak width, and the frequency dependence of the peak width.
Accordingly, it is an object of the present invention to provide a method for noninvasively determining the liquid level for a variety of stored liquids.
Another object of the present invention is to provide a method for determining the level of liquids stored in a variety of containers.
Yet another object of the invention is to provide a method for noninvasively determining the liquid level for flammable stored liquids.
Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.